Air Force says GPS system won’t fail, though risk exists

The United States Air Force has issued a response to the report given earlier this week by the Government Accounting Office (GAO) to Congress (see Geek.com’s coverage). In that report a degree of uncertainty was conveyed about whether or not the Air Force would be able to maintain GPS accuracy in the next decade, due to a shortage of available satellites — as some of the older ones exceed their operational lifetime and begin to fail.

The Air Force’s response states that while a risk does exist (that the GPS system could fail in some ways, resulting in decreased accuracy), that risk is “very small”.

Lt. Col. Tim Lewallen said there are currently 30 working satellites in orbit, plus three older satellites that could be reactivated if needed. Another satellite, based on the previous generation of GPS technology, is due to go up in August. And Space Command reports the $30 billion system has never been larger or more accurate than it is today.

A fully encompassing system requires at least 30 satellites for extremely accurate global coverage, though any system can operate with reasonable accuracy with as few as 24 or 25. A typical GPS device is accurate to within 30 feet. Commercial and military grade GPS units can cost well over $1000 each, and with ground-based assist signals, include accuracy to within millimeters.

Per Enge, a professor of Aeronautics and the director of the GPS Research Laboratory at Stanford University said the Air Force has developed stopgap measures, but they won’t work forever:

No one can complain or state that the sky is falling right now. The GPS constellation is skinny compared to what it should be. The most important thing is that we keep funding GPS and don’t take it for granted.

While there are at least two other functional GPS systems available right now, the United States maintains its own GPS system. There is also a Russian GLONASS (Global Navigation Systems Satellite) system, which had deployed 29 satellites in orbit as of 2007. The European Galileo system has 30 satellites in orbit, and will be fully operational by 2012. A third, the Chinese BNS (Beidou Navigation System) has already launched five satellites which, today, operate over mainland China. The system will have 30 satellites in orbit, along with spares, by the year 2015.

It is unlikely we need to worry as these systems, which while not operating identically to the US GPS system (meaning the same frequencies/signals), do provide global coverage. We may just have to buy a European, Russian, or China-compatible device.

Anecdotally, in late 2008, I spoke with a land surveyor in Wichita, Kansas about the GPS device he was carrying with him. It was the only piece of equipment he brought into the hotel with him each night. One day I asked him why that was? He told me of the device’s expense, importance to his company, and its sensitivity, which included repeatable measurements at the same location to within a few millimeters, as sampled over the course of several hours and even days on repeat/return trips.

Commercial applications like this allow for repeated measurements from permanent stakes driven into the ground at property boundries. The device is rested on the stake in a particular way, and readings are taken. As with consumer GPS units, the device takes only a couple seconds of being motionless to provide its reading. It included a built-in computer for determing area, mapping, and other data exchange abilities. Quite an impressive piece of mobile technology — about the size of three conventional notebook computers stacked on top of each other, weighed about 15 lbs total.